The actions of metabolic inhibition on human detrusor smooth muscle contractility from stable and unstable bladders

Objectives To determine the important cellular site(s) of action of a brief exposure to NaCN (chosen to reduce mitochondrial respiration and hence mim ic cellular hypoxia) on the mechanical properties and regulation of intrace llular [Ca2+] in human detrusor smooth muscle. Using muscle samples obtaine d from patients with stable and unstable bladders, to determine whether the unstable bladder is associated with changes in the functional properties o f detrusor muscle under these circumstances. Materials and methods Experiments were conducted in vitro on muscle strips or isolated cells. Isometric tension was recorded in muscle strips during e lectrical stimulation or exposure to agonists. Intracellular [Ca2+] and [H] were measured by epifluorescence microscopy, and cell autofluorescence me asured as an index of mitochondrial function. Results There were no differences in the responses to electrical stimulatio n and varying concentrations of carbachol in muscle strips from stable and unstable bladders. NaCN (2 mmol/L) reduced the contraction induced by carba chol (10 mu mol/L) by a mean (sd) of 43 (16)% and 56 (15)% in the two group s; the reduction in the unstable was significantly less than in the stable group. NaCN similarly reduced the response to 10 mmol/L caffeine, but had n o effect on the KCl-induced contraction. NaCN significantly increased the r esting sarcoplasmic [Ca2+] and attenuated the calcium transients evoked by carbachol and caffeine, but again had no effect on the KCl-induced transien t. The reduction of the carbachol calcium transient was also less in cells from unstable bladders than in those from stable bladders. There was no eff ect of NaCN on intracellular pH, except for a brief, transient alkalosis. Conclusions NaCN reduces both the contraction and Ca-transient to carbachol by reducing Ca2+ accumulation by intracellular stores, because the carbach ol- and caffeine-evoked responses were similar. Any effect on transmembrane Ca2+ flux was minimal because there was no effect on KCl-induced responses . The greater resilience of tissue from unstable bladders to acute cellular hypoxia may reflect some adaptation acquired in vivo.